bpf: fix uapi hole for 32 bit compat applications

In 64 bit, we have a 4 byte hole between ifindex and netns_dev in the
case of struct bpf_map_info but also struct bpf_prog_info. In net-next
commit b85fab0e ("bpf: Add gpl_compatible flag to struct bpf_prog_info")
added a bitfield into it to expose some flags related to programs. Thus,
add an unnamed __u32 bitfield for both so that alignment keeps the same
in both 32 and 64 bit cases, and can be naturally extended from there
as in b85fab0e.

Before:

  # file test.o
  test.o: ELF 32-bit LSB relocatable, Intel 80386, version 1 (SYSV), not stripped
  # pahole test.o
  struct bpf_map_info {
	__u32                      type;                 /*     0     4 */
	__u32                      id;                   /*     4     4 */
	__u32                      key_size;             /*     8     4 */
	__u32                      value_size;           /*    12     4 */
	__u32                      max_entries;          /*    16     4 */
	__u32                      m...

include/uapi/linux/bpf.h

@@ -1017,6 +1017,7 @@ struct bpf_prog_info {
 	__aligned_u64 map_ids;
 	char name[BPF_OBJ_NAME_LEN];
 	__u32 ifindex;
+	__u32 :32;
 	__u64 netns_dev;
 	__u64 netns_ino;
 } __attribute__((aligned(8)));
@@ -1030,6 +1031,7 @@ struct bpf_map_info {
 	__u32 map_flags;
 	char  name[BPF_OBJ_NAME_LEN];
 	__u32 ifindex;
+	__u32 :32;
 	__u64 netns_dev;
 	__u64 netns_ino;
 } __attribute__((aligned(8)));

tools/include/uapi/linux/bpf.h

@@ -1017,6 +1017,7 @@ struct bpf_prog_info {
 	__aligned_u64 map_ids;
 	char name[BPF_OBJ_NAME_LEN];
 	__u32 ifindex;
+	__u32 :32;
 	__u64 netns_dev;
 	__u64 netns_ino;
 } __attribute__((aligned(8)));
@@ -1030,6 +1031,7 @@ struct bpf_map_info {
 	__u32 map_flags;
 	char  name[BPF_OBJ_NAME_LEN];
 	__u32 ifindex;
+	__u32 :32;
 	__u64 netns_dev;
 	__u64 netns_ino;
 } __attribute__((aligned(8)));